The recent agricultural technology is, both at home and abroad, directed to environment-protecting agriculture, and it is desired to establish agricultural techniques that are gentle to ecology. Given the situation, it is an urgent necessity for us to take some measures to prevent soil-borne diseases in raising crop plants and also flowering plants, but there are known no agricultural chemicals that are effective for preventing such soil-borne diseases without polluting the environment, and it has been difficult to prevent soil-borne diseases.
Heretofore, in plant husbandry under structure or the like, large amounts of soil fumigants have been being used for soil disinfection in order to prevent soil-borne diseases to be caused by injuries in continuous cropping or the like, thereby ensuring the producibility therein.
However, such soil fumigants are problematic in that they have negative influences on ecology including human beings, and therefore could not be used in future. Accordingly, it is now an important theme in the art to develop any other materials that are gentle to ecology for preventing soil-borne diseases, in place of soil fumigants.
On the other hand, to promote the orientation toward environment-protecting agriculture, many attempts have been being made at the use of materials comprising microbes that grow in soil and produce therein antagonists (antifungals and antibacterials against phytopathogens) thereby to prevent soil-borne diseases. However, such microbial materials are problematic in that their applicability is limited, that the reproducibility of their effects is often poor, that their effects do not often last long, and that their use is often difficult.
Given the situation as above, we, the present inventors have been studying various measures for preventing soil-borne diseases by the use of various microbial materials. We have already disclosed a means of using fluorescent bacteria that grow in the endorhizosphere of crop plants for preventing soil-borne diseases (see Japanese Patent Application Laid-Open No. 7-163334).
In addition, we have already proposed seeds as treated with said fluorescent bacteria along with N-acyl-lactams, in order to ensure the prevention of soil-borne diseases of crop plants thereby promoting the growth of crop plants (see Japanese Patent Application No. 7-23515).
Further, we have already proposed a material for preventing bacterial wilt, which comprises fluorescent bacteria with the ability to produce crystalline 2,4-diacetylphloroglucinol and with no antibiotic tolerant, said bacteria being isolated essentially from the endorhizosphere of crop plants (see Japanese Patent Application No. 7-99628).
Moreover, we have already proposed a method for preventing bacterial wilt, which comprises using microbes with phenol tolerance and with no ability to produce antimicrobial materials, said microbes being isolated essentially from the endorhizosphere of crop plants (see Japanese Patent Application No. 7-99629).
We have confirmed that the above-mentioned fluorescent bacterial materials comprising fluorescent bacteria that grow in the endorhizosphere of crop plants are effective for preventing bacterial wilt of crop plants that are raised within the range of temperatures at which said fluorescent bacteria can grow. However, we have found that the materials could not satisfactorily exhibit their effects in plant husbandry under structure or the like where the temperature may be 40.degree. C. or higher. Under such high-temperature conditions, the colonization of the fluorescent bacteria in the endorhizosphere of crop plants is greatly lowered, resulting in that the fluorescent bacteria could not satisfactorily exhibit their effects in crop fields where the degree of severity of bacterial wilt is high or in long-term cultivation of crop plants.
With the recent development in intensive agriculture for plant husbandry under structure, the frequency of soil-borne diseases is increasing. For example, composite diseases of bacterial diseases and fungal diseases are increasing. However, even in crop fields with such composite diseases, some plants can still grow well. We, the present inventors have specifically noted this fact and have further promoted our studies. Specifically, we have searched healthy plants that are growing around the plants having soil-borne diseases, for gram-negative bacteria which are presumed to have the highest compatibility with plant roots.
After having succeeded in finding out the intended gram-negative bacteria, we have screened them for selecting therefrom those capable of colonizing in the endorhizosphere of plants to exhibit their function of inhibiting or preventing soil-borne diseases such as bacterial wilt, and have obtained two strains capable of endosymbiotically and mutualistically colonizing in the endorhizosphere of plants of Solanaceae with high frequency. We have further studied the means of incubating said two strains and the means of applying them to crop fields, and as a result, have found that, when said two strains are introduced into the endorhizosphere of plant seedlings, then they endosymbiotically and mutualistically colonize not only in the endorhizosphere of plants of Solanaceae but also in that of plants of Cruciferae, strawberries, potatoes, carnations and others, and the seedlings growing endosymbiotically with said two strains exhibit strong resistance against bacterial and fungal soil-borne diseases. On the basis of these findings, we have completed the present invention.